A lighting device is known from EP 1 742 168 A1. Such lighting devices are part of optoelectronic sensors, as they are for example used in the logistics for detecting codes, like for example bar codes or QR-codes on packages. Application fields of such code readers with lighting devices include package sorting and package tracking for parcel and currier services, identification systems in automatic storages, in a luggage sorting system at airports, product loading and unloading systems and in the quality inspection of objects.
Such optoelectronic code readers have cameras, preferably line scan cameras. Packages on conveyor belts pass said cameras. A lighting device is required, which has a line-like illumination in direction of the detection area of the line scan camera. In this case, a homogeneous brightness distribution is especially advantageous within the illuminated line or line of light to ensure the exact detection of the code for the line scan camera.
EP 1 742 168 A1 is concerned with the problem of common arrangements, which have LEDs for the illumination of the detection area. These arrangements do not have additional beam forming (optical) elements and thus, have especially at large distances between the light sources and the area of the illumination a low efficiency or low brightness and furthermore, do not ensure a uniform illumination.
To solve this problem, according to EP 1 742 168 A1 beam forming optical condenser lenses are provided, which are formed the same in the whole arrangement and have a planar face, facing the light source, and on the opposite side have a spherical, aspherical or toric surface. With increasing distance to the receiver optics the LEDs have a different off-axial alignment to the optical axis of the respective condenser lens. By means of this special arrangement of the off-axial alignment of the LEDs to the optical axis of the respective condenser lenses, the main beam direction of the light, radiated from the LEDs, can be influenced such, that a very special illumination profile is produced. In this manner, for example, the edge decrease of the receiver lens can be compensated. The condenser lenses and the cylinder lens form together an anamorphic projection lens, which forms the light of each of the LEDs to one line of light. In this case, each LED forms a single line of light, wherein the lines of light of the several LEDs is superimposed in a detection range to a resulting line of light. The type and manner of the superimposition of the individual lines of light is influenced such, that the light distribution can vary in the resulting line of light.
DE 10 2008 019 084 A1 discloses also a lighting device, which illuminates a narrow, elongated area with high intensity. The device has a plurality of LEDs arranged adjacent to each other on a flat carrier along a line. A conversion device is arranged with a distance to the LEDs in direction of an optical path and extends along the line of LEDs. Said conversion device converts light beams radiated from the LEDs by means of a plurality of light deflecting elements into light beams, which angle distribution is mixed up in at least one plane. Especially diffusing, breaking or diffractive elements are used as light deflecting elements. The light deflection of the light beams radiated by the individual LEDs is carried out such, that a direct visibility of the individual LEDs, when viewing with a camera, is prevented. To achieve a narrow illuminated area, a condenser lens is provided in the optical path behind the light deflecting elements, focusing the light on one strip in order to achieve a line of light.